This invention relates to measuring system frequency and more particularly to a means and method for measuring power system frequency deviation from a predetermined reference frequency without measuring the time between zero crossings of the voltage waveform. The system may be an electrical power system, a rotating mechanical system or any other system experiencing periodic motion from which a parameter representative of the periodic motion is available. Electrical power systems are discussed generally herein due to their widespread applicability.
Mechanical frequency limits are established to protect an electrical generator prime mover, typically a turbine; it is generally undesirable to operate a turbine above or below predetermined mechanical resonance limits or frequencies. Thus, it is important to determine the operating frequency of the turbine. Typically electrical signal frequency, indicative of turbine mechanical frequency, is monitored and electrical frequency limits are predetermined to ensure that the turbine operates within the permissable band of mechanical frequencies.
Further, before interconnecting loads supplied by different generators, it is necessary to ensure that both generators are operating at the same electrical frequency in order to prevent voltage surges and voltage transients which may damage the loads should interconnection of non-synchronous generators be attempted.
Previously known electrical power frequency and frequency deviation measuring techniques typically employ means for measuring the time between zero crossings of the voltage waveform and generating frequency and/or frequency deviation values from these measurements. These methods are sensitive to noise on the power lines since voltage spikes or glitches may be detected as zero crossings of the voltage waveform thereby leading to erroneous frequency determinations. Further, non-fundamental components may produce signals detected as zero crossings which could also produce erroneous frequency results.
Improper or erroneous frequency determinations may lead to costly premature system shutdown if the system frequency does not actually exceed the predetermined frequency limits and the frequency monitor indicates it does exceed the limits or may result in damage to the prime mover and other system equipment if the system frequency does actually exceed the predetermined limits and the frequency monitor indicates it does not exceed the limits. Further, non-synchronous generators may be interconnected, resulting in problems hereinbefore described, if the frequency monitor erroneously indicates that the frequencies of the generators are the same.
A recent article, "A New Measurement Technique for Tracking Voltage Phasors, Local System Frequency, and Rate of Change of Frequency," A. G. Phadke et al., IEEE Transaction on Power Apparatus and Systems (May, 1983), purports to determine power system deviation. The authors present a technique which uses the sum and difference of two vectors, with an assumption that the vectors are perpendicular. This assumption typically leads to erroneous results since the vectors are generally not perpendicular. In fact, they are perpendicular only when the reference frequency is equal to the power system frequency. It is estimated that the present invention is 3-10 times more accurate than that disclosed by Phadke et al.
Accordingly, it is an object of the present invention to provide a means and method for measuring system frequency which do not employ a zero crossing technique.
Another object is to provide a means and method for measuring system frequency which do not respond to non-fundamental signal components.
Yet another object is to provide a means and method for accurately determining system frequency and frequency deviation.